1. Field of the Invention
This invention relates to ion transporting membranes for use in proton exchange processes.
2. Description of the Prior Art
Ion exchange membranes have many ionizable groups in their polymeric structure. In these membranes, one ionic component of these groups is retained by the polymeric matrix of the membrane while another ionic component remains mobile, consisting of a replaceable ion which is electrostatically associated with the ionic component retained by the polymeric matrix. The characteristic ability of the mobile ion to be replaced under appropriate conditions by other ions imparts the ion exchange characteristics of these membranes. Ion exchange membranes can be either cation exchange or anion exchange membranes. Ion exchange membranes are outstanding separators for use in electrochemical cells since these membranes are permeable to one kind of ion while resisting the passage or direct flow of liquids and ions of opposite charge. The membranes can be made self supporting and can also be reinforced so as to produce membranes having high mechanical strength. The thickness of the membranes is preferably as small as possible, for example, from about 0.05 to 1 mm.
The use of ion exchange membranes in the solid polymer electrolyte cells, fuel cells, and water electrolysis cells of the prior art involves the use of a solid polymer sheet ion exchange membrane as the sole electrolyte. The polymer sheet also acts as the cell separator. The present applications use permselective membranes having a thickness of about 0.1-0.5 mm. Prior to use in the cell, the membrane, for instance, NAFION.RTM. 117, sulfonated perfluorocarbon polymer, is hydrated in water so as to contain approximately 30% water based upon the dry weight of the cell membrane. The hydration process enhances the membrane conductivity to hydrogen ions. Electrochemical cells employing solid polymer electrolyte cell membranes usually have a bipolar configuration. In such cells, the electrocatalysts are bonded to each side of the membrane and the resulting solid polymer electrolyte is a structurally stable membrane and electrode assembly.
Fuel cells convert chemical energy directly into electrical energy without going through a heat cycle. The overall conversion to electrical energy is more efficient than in conventional power sources. The many different types of fuel cells can be characterized generally by the medium by which an ionic species migrates from an anodic chamber to a cathodic chamber. Examples of different types of fuel cells are the solid oxide, phosphoric acid, alkaline, molten carbonate and the polymeric membrane.
In fuel cells having a polymeric membrane which is ion permeable, for instance a 5 mil. thick perfluorosulfonic acid membrane having a catalyst coating sandwiched between the bipolar cells, molecular hydrogen enters the anodic compartment and oxygen or air enters the cathodic compartment. Hydrogen is oxidized to produce protons and electrons and the oxygen is reduced. The protons are driven through the thin gas separating membrane by the potential gradients developed at each electrode. The proton reacts with the oxygen or oxide species to form water. The electrons are driven through the circuitry and through the load and then returned to the cathode.
Current commercial applications of fuel cells having polymeric membranes are limited and include military applications and low power hydrogen/air operation. The number of commercial applications is limited by the high cost of the system. It is apparent that improvements in efficiency of fuel cells would increase the number of commercial applications. Typically, fuel cells generate power using catalyst coated membranes in a range of 0.32 to 0.5 kw per square foot of catalyst coated membrane utilized in the cell. Useful fuel cells combine numerous single cells in series so as to increase voltage.
Water electrolysis cells are similar to fuel cells, but instead of operating the cell galvanically, electrical power is added to the cell together with water. The water electrolysis cell is, thus, simply a membrane fuel cell which is operated in reverse. The essential feature of a fuel cell having an ion exchange membrane is the membrane and electrode assembly. The ion exchange membrane is generally a co-polymer of tetrafluoroethylene and a vinyl ether monomer containing a functional group. The membrane is characterized by the ion exchange capacity thereof. This is commonly referred to in terms of the equivalent weight of the ion exchange membrane. The equivalent weight is a property capable of measurement which is related to the ion transporting capability of the ion exchange membrane. In addition to the equivalent weight of the ion exchange membrane, co-polymers of tetrafluoroethylene and a vinyl ether monomer having a functional group attached thereto have generally excellent gas separation characteristics, good ionic conductivity, reasonable mechanical strength, and good handling characteristics.
Although the ion exchange membrane materials of the prior art have been used in fuel cells, their usefulness is reduced because of the low power output and high dependency for efficiency on the membrane and the presence of imbibed water therein. For the greatest usefulness of an ion exchange membrane, the power output in cells containing such membranes should be as high as possible, specifically, being capable of retaining high voltages as the load or current density is increased. Regardless of the increased performance levels upon use of ion exchange membranes, additional voltage gains at a fixed current density would be desirable. In a fuel cell, a proton which is generated by the oxidation of molecular hydrogen is solvated by the water which is added to the fuel cell by a humidification system. The solvated proton is passed through the ion exchange membrane. If the fuel cell humidification system, consisting of water added to the incoming gas stream, is insufficient to solvate the protons, then the source of water for solvation of the proton must become the hydrated membrane itself. The water is available as water which is associated with the sulfonic acid functional groups in the ion exchange membrane. This condition can lead to cell failure if allowed to persist because dehydration of the ion exchange membrane will eventually occur, resulting in increased electrical resistance in the ion exchange membrane, which is manifested by heat build-up.
The use of ion exchange membranes having at least one electrode bonded to a surface of the membrane in electrolysis cells is now well known and such cells are typically illustrated in U.S. Pat. Nos. 4,191,618; 4,212,714, and 4,333,805, assigned to the General Electric Company. In water electrolysis cells and fuel cells, the electrodes are attached by the application of heat and pressure to a membrane in the hydrated form. The functional groups which permit cation transport may be sulfonates, carboxylates or phosphonates. These functional groups are attached to a polymeric, and preferably, a perfluorinated polymer backbone.
In general, the polymers have found most widespread use in the above applications when the functional group is on a fluorocarbon chain which is pendant to the main polymer backbone. Fluorocarbon sulfonic acid polymers and carboxylic acid polymers have been disclosed in the prior art which have the functional group attached directly to the backbone, but these polymers have found scant utility (U.S. Pat. No. 3,041,317 and British Pat. No. 1,497,748). The polymer materials, whether based upon fluorocarbon sulfonic acids or carboxylic acids, have in general been made by co-polymerizing monomers such as tetrafluoroethylene or chlorotrifluoroethylene with fluorocarbon vinyl ethers which contain an acid or an acid precursor functional group (U.S. Pat. No. 3,282,875 and Brit. Pat No. 1,518,387).
The relationship between water absorption of the polymer forming the membrane and usefulness of the polymer as a membrane has long been recognized (W. G. F. Grot, et. al., Perfluorinated Ion Exchange Membranes, 141st National Meeting, The Electrochemical Society, Houston, Tex., May, 1972). Grot disclosed that the capacity of the polymer to absorb water is a function of the equivalent weight of the polymer, the history of pretreatment of the polymer and the electrolytic environment of the polymer. The equivalent weight is the weight of polymer which will neutralize one equivalent of base. It is a measure which can be correlated with the ionic conductivity of the membrane. A standard method of measuring water absorption for meaningful comparisons is given in Grot's paper (above). The method consists of boiling the polymer for 30 minutes in water with the polymer being in the sulfonic acid form. The water absorbed by the polymer under these conditions is called the "Standard Water Absorption". The sulfonic acid based polymer membranes reported on in Grot's paper are composed of the polymers disclosed in U.S. Pat. No. 3,282,875.
One way of correlating functional groups to performance is to measure water of hydration per functional group in the polymer. Comparison of polymers containing carboxylic acid derived functionality (U.S. Pat. No. 4,065,366) and polymers containing sulfonamide derived functionality shows that the carboxylic acid type hydrate less than the sulfonamide type according to C. J. Hora, et al., NAFION, sulfonated perfluorocarbon polymer, Membranes Structured for High Efficiency Chlor-Alkali Cells, 152nd National Meeting The Electrochemical Society, Atlanta, Ga., October, 1977. Changes in functional group concentration in a given polymer structure results in changes in the hydration water per functional group. Thus, Hora disclosed that a 1500 equivalent weight sulfonic acid polymer of given structure has less water of hydration per functional group than an 1100 eq. wt. polymer of the same general structure. In turn, the electrical resistance of the 1500 eq. wt. material is higher than the 1100 eq. wt. material because of the availability of fewer sites to transport ions and thus to conduct current. Sulfonic acid type membranes which are useful in fuel cells are taught in the prior art to have eq. wts. in the range of 1100 to 1200. In practice, eq. wts. of about 1100 are considered best because of lower electrical resistance.
Data for the dehydration of sulfonic acid polymers and sulfonamide polymers has been published by Hora and Maloney in the above publication. In this paper, the polymer structures are the same except for the substitution of sulfonamide groups for sulfonic acid groups. The data shows that, for given eq. wts., the sulfonamides absorb only 35-60% as much water as do the sulfonic acids. A particular case shown is a comparison of 1200 eq. wt. membranes. There, the sulfonic acid membrane absorbs about 20 moles of water per equivalent of sulfonic acid, while the sulfonamide, from methylamine, absorbs 12.3 moles of water per equivalent of sulfonamide and the sulfonamide, from ethylenediamine, absorbs only 8.1 moles of water per equivalent of sulfonamide. From another paper (H. Ukihashi, Ion Exchange Membrane For Chlor-Alkali Process, Abstract No. 247, American Chemical Society Meeting, Philadelphia, Pa., April, 1977) it can be calculated that a carboxylic acid membrane having an eq. wt. of 833 absorbs 8.3 moles of water per equivalent of carboxylic acid and that another having an equivalent weight of 667 absorbs 9.2 moles of water per equivalent of carboxylic acid. Until this invention, substantially greater water absorption of the hydrated membrane than 30-40% by weight, based upon dry weight of the membrane, has not been obtained.
Solid polymer electrolyte catalytic electrodes are used in various devices and processes. For example, they are used in fuel cells, gas generating devices, processes for chemical synthesis, devices for chemical treatment and gas dosimeters and sensing devices and the like. Solid polymer electrolyte catalytic electrode assemblies are currently manufactured by several techniques. U.S. Pat. No. 3,297,484 illustrates in detail materials for electrode structures including exemplary catalyst materials for electrodes, ion exchange resins for solid polymer electrolyte permselective membranes and current collecting terminals. Catalytically active electrodes are prepared in the prior art from finely divided metal powders mixed with a binder, such as polytetrafluoroethylene resin. A typical solid polymer electrode assembly comprises a bonded structure formed from a mixture of resin and metal bonded upon each of the two major surfaces of a solid polymer electrolyte permselective membrane. In U.S. Pat. No. 3,297,484, the resin and metal or metal alloy powder mix is formed into an electrode structure by forming a film from an emulsion of the material, or alternatively, the mixture of resin binder and metal or metal alloy powder is mixed dry and shaped, pressed and sintered into a sheet which can be shaped or cut to be used as the electrode. The resin and metal powder mix may also be calendered, pressed, cast or otherwise formed into a sheet, or a fibrous cloth or mat may be impregnated and surface coated with the mixture of binder and metal or metal alloy powder. In U.S. Pat. No. 3,297,484, the described electrodes are used in fuel cells. In U.S. Pat. No. 4,039,409, the bonded electrode structure made from a blend of catalyst and binder is used as the electrode in a gas generation apparatus and process.
In U.S. Pat No. 3,134,697, many ways are described for incorporating catalytically active electrodes into the two major surfaces of a permselective ion exchange resin membrane. In one method, the electrode material made of metal or metal alloy powder and a resin binder may be spread on the surface of an ion exchange membrane or on the press platens used to press the electrode material into the surface of the ion exchange membrane. The assembly of the ion exchange membrane and the electrode or electrode materials is thereafter placed between the press platens and subjected to sufficient pressure, preferably at an elevated temperature, sufficient to cause the resin in either the membrane or in admixture with the electrode material either to complete the polymerization, if the resin is only partially polymerized, or to flow, if the resin contains a thermoplastic binder.